This invention concerns analogue displays, for example timepieces (ie watches or clocks) and analogue meter displays having dial, arc or rectilinear scales where a scalar quantity is represented by the relative position of two indices against an optically contrasting background.
Analogue watches and analogue meter displays have typically been of either mechanical or electromechanical construction. However, one example of a display of non-mechanical construction, a liquid crystal device analogue watch having a radial display format, has recently been described (cf. Conference Record of the IEEE Biennial Display Research Conference Oct. 24-26, 1978, pp 59-61). As there described, a set of electrodes of conventional meander configuration overlap inner and outer spaced sets of segment electrodes across a liquid crystal cell and are addressed using 1/2-duty cycle time-multiplexing to allow the simultaneous display of both hour and minute function indices. By appropriate electrical address the voltage V.sub.on across electrodes defining the index position, in each case, is of such value above a threshold value V.sub.th, characteristic of the liquid crystal material, that a localized region of the liquid crystal material is switched ON and adopts a state providing optical contrast with the adjacent and remaining parts of the display where voltage differences V.sub.off less than but near threshold are applied. This allows the number of connections to the display to be reduced compared to the number required to make individual connection to each directly driven active area of the display.
The performance limits of liquid crystal displays at a given temperature are determined by the values of the voltage differences V.sub.on, V.sub.off applied. It is desirable for good optical contrast that the voltage difference V.sub.on approaches or is greater than the saturation voltage difference V.sub.sat required to drive the optical response of the liquid crystal display into the fully ON state, while at the same time it is necessary, for effective operation, that the voltage difference V.sub.off is at most less than or equal to the threshold voltage characteristic of the display. Further limitations arise, however, because both the threshold voltage V.sub.th and the saturation voltage V.sub.sat, characteristic of the display, vary with temperature. They may also vary with the angle of view. It is desirable, therefore, that the ratio of the R.M.S. average voltage differences--R=&lt;V.sub.on &gt;RMS/&lt;V.sub.off &gt;RMS is optimized to be as large as possible.
The best value of this ratio R that has been achieved for two-function display time-multiplexed devices is .about.2.25 (cf. Conference Record of the IEEE Biennial Display Research Conference Oct. 24-26, 1978, pp 59-61.)
The problems encountered with electro-luminescent panel displays and gas discharge displays are in many respects similar to those referred to above.
One approach to these problems of index display has been disclosed at the Seminar on Liquid Crystal Devices, San Jose, 7-8 February 1979. As there described, pseudo-random coded binary voltage signals are applied, after appropriate selection, to a set of electrodes of modified meander configuration, and to a set of segment electrodes. The voltage signals are applied so that the display is maintained, at selected index positions, in the OFF state, corresponding to an applied zero voltage difference, while all other regions of the display are maintained in the ON state. With this approach it is possible to achieve high (even infinite) values of the ratio R and to extend the performance limits of analogue displays. However, though this approach is satisfactory for many applications, it can have a number of drawbacks. Relatively high drive voltage signal levels may be required, and the spacings between electrodes can result in an undesirable visible background pattern. Also, when a liquid crystal medium containing pleochroic dye is used, it is frequently only possible to display the indices as dark characters (OFF state) against a light background (ON state). This is certainly the case where this technique disclosed is used for a display panel including a layer of cholesteric liquid crystal material of positive dielectric anisotropy with pleochroic dye, the panel being arranged as a dyed cholesteric-to-nematic phase change effect device. Reverse effects ie light characters (OFF state) against a dark background (ON state) could be provided by other dyed liquid crystal display panels known in the art--eg panels providing hometropic alignment of dyed long pitch cholesteric material, which material exhibits--ve dielectric anisotropy. In such panels the liquid crystal material spontaneously adopts a nematic phase (OFF state, light) and is driven upon application of an appropriate electric field across the panel, with a cholesteric planar texture (ON state, dark). The contrast and brightness of such panels, however, is, in general, inferior to that obtained for dyed cholesteric-to-nematic phase change effect devices.
An alternative approach to the problems of two-index character display is described below. It is an advantage of the invention that the attainable contrast and brightness is in general better than that provided by displays having time multiplexed address.